The Study On Phase Transformation Of Hardening Precipitates In Al-Cu-Li-Mg Aluminum Alloy During Aging

Al-Cu-Li alloys are one of the most important lightweight structure materials. Owing to their relatively low density, high specific strength and elastic modulus, combined with balance of fracture toughness at low temperature and fatigue properties and good corrosion resistance, they are widely used as structural materials in aviation and aerospace industry. Every 1wt.% of Li added to the alloy reduces the total density of the alloy by 3% and increases the elastic modulus by 6%. With addition of a small amount of alloy elements, such as Cu, Mg and Ag, a great number of precipitates with atomic scale can be formed in Al matrix after proper aging treatments. The number and size of these precipitates are different under different aging treatments, which results in the difference of properties of the alloy. Therefore, it is important to understand the properties of the alloy by means of microstructures. However, the single-nanometer sized precipitates are very difficult to be accurately characterized for their structures by conventional methods, such as X-ray diffraction, electron diffraction and electron microscopy imaging without a sufficient resolution. Hence, the problems about the structures of main hardening precipitates and the precipitated process have thus far not yet been adequately solved. Recently, the development of atomic-resolution electron microscopy has made it feasible to fully understand the evolutions of the precipitates well at the atomic scale.Using advanced Cs-corrected and conventional electron microscopy, this dissertation aims to determine the atomic structure of the main har dening precipitate T₁ phase in Al-4.15Cu^-1.15Li-0.29Mg（wt.%） alloy by means of quantitative image analysis and high angle annular dark-field（HAADF）- scanning transmission electron microscopy（STEM） technique, in association with the first principle calculations. The evolution of precipitates of the alloy during single aging treatment and double ageing treatment have also been studied in the alloy. The determined precipitate structure and the evolution mechanisms have been used to understand the relationship between process and property in the alloy system.The main contents and conclusions of this dissertation are summarized as below:（1） Microstructure evolution of the target alloy aged at 80 ℃ ²00 ℃ was systematically studied using HAADF-STEM technique in combination with conventional TEM and selected area electron diffraction technique. And the precipitates formed during different aging conditions and the relationship between precipitates and hardness has also been studied. It was found that the main precipitates formed during 80℃¹35℃ are GP zones and δ′-（GP zones）-δ′ composite phases. The structure of GP zones are main conventional single-atomic-layer GP I zones. Small amount of GP zones are two-atomic-layer or one or two Al atomic layers existing between the two Cu atomic layers. With increasing of aging temperature, the two phases coarsen. The precipitates during peak-ageing at 145℃ are GP zones and δ′-（GP zones）-δ′ composite phases and GP_T1 zones. δ′-（θ′）-δ′ composite phases, σ phases, GPB and T₁ phases are the main precipitates in the peak-aging at 165℃, 180℃ and 200℃.（2） The atomic structure and structure evolution of T 1 precipitate during aging at 165℃ were studied by means of HAADF-STEM combined with the first-principle energy calculations. It is the first time to report that T₁ precipitates derive from their own GP zones, termed as GP_T1 zones The atomic structure of GP_T1 zones was studied by means of Cs-corrected TEM and HAADF-STEM image simulation. The GP_T1 zone is coherent with Al matrix. Two Al atomic layers in the matrix were replaced by two Cu-rich layers. A ‘sandwiched’ structure lying on {111}Al formed. The distance between the Cu-rich layers is 0.466 nm, which is smaller than that of T 1 phase（0.495nm）. The formation enthalpy of metastable precipitates during the transformation from GP_T1 zone to T₁ phase is respectively-6.8k J/mol·atom^-1、^-19.0k J/mol·atom^-1、-21.3k J/mol·atom^-1、-32.2k J/mol·atom^-1, indicating the transformation is also energetically reasonable.（3） A variant of T₁ phase was observed firstly in the alloy. The variant of T₁ phase consists of two misaligned T₁ unit cells in thickness. There are a space of d 111 Al between the upper and lower unit cells, and also, the upper ones mo ves <112>Al/6 in relation to the lower one. The variant of T₁ phase can exist as individual, and also can nucleate on the interface between T₁ phase and the matrix. The calculation results show that the formation enthalpy of the variant of T 1 phase was calculated to be-32.7 k J/mol·atom^-1, indicating that the structure of variant of T₁ phase proposed is energetically reasonable.（4） T₁ precipitates coarsens in two paths. One is regular by the nucleation of GP_T1 zone on one side of matured T₁ precipitate along its c-axis in proper Li area. The other is abnormal via the variant of T₁ phase in excess Li area in order to consume excess Li. The transformation from GP_T1 zones to T₁ precipitates was observed in both of the two paths.（5） The effect of natural aging on precipitation process and hardness during later artificial aging was studied by HAADF-STEM technique and hardness test. Natural aging for 7 days has negative effect on the hardness of the alloy. In contrast, natural aging for 60 days has positive effect on the hardness of the alloy. The peak-hardness of samples kept in natural ageing for 60 days before artificial ageing at 180℃ and 200℃ can keep longer in compare with sample without natural ageing or artificial ageing for shorter times. The precipitation sequence in the alloy two-step-aged at 180℃ is: GP I + δ′-（GP I）-δ′ → GPB + σ + GP_T1 + S → GPB + T₁+ S → T₁+ S. The precipitation sequence in the alloy two-step-aged at 200℃ is: GP I + δ′-（GP I）-δ′ → GPB + σ + GP_T1 + S → σ + T₁ + S → T₁ + S. Excess Li would segregate at the interface of S phase in the under-aging at 200℃,whcich makes S phase more stable. The main hardening T₁ precipitates also derive on their own GP_T1 zones and coarsen by the nucleation of GP_T1 zone on one side of matured T₁ precipitate during double aging.